Fix --diff output when encountering EOF

`split("\n")` includes a final empty element `""` if the final line
ends with `\n` (as it should for POSIX-compliant text files), which
then became an extra `"\n"`.

`splitlines()` solves that, but there's a caveat, as it will split
on other types of line breaks too (like `\r`), which may not be
desired.

Fixes psf#526.

black.py

@@ -3876,8 +3876,8 @@ def diff(a: str, b: str, a_name: str, b_name: str) -> str:
     """Return a unified diff string between strings `a` and `b`."""
     import difflib
 
-    a_lines = [line + "\n" for line in a.split("\n")]
-    b_lines = [line + "\n" for line in b.split("\n")]
+    a_lines = [line + "\n" for line in a.splitlines()]
+    b_lines = [line + "\n" for line in b.splitlines()]
     return "".join(
         difflib.unified_diff(a_lines, b_lines, fromfile=a_name, tofile=b_name, n=5)
     )